1. Field of the Invention
This invention relates to a protective agent for the surface of a lithographic printing plate, especially to an emulsion type plate protective agent.
2. Description of the Prior Art
The final step in the process of making a lithographic printing plate is generally referred to as a gumming step since in this step, the plate is coated with a gumming solution to protect the non-image area (which retains water and repels greasy printing ink). While the primary purpose of gumming is to desensitize, or increase the hydrophilicity of the non-image area of the lithographic printing plate, it has other purposes: that is, to prevent possible deterioration of the lithographic printing plate during the period between making the plate and printing, or over a period in which printing is interrupted; to prevent the non-image area from becoming receptive to printing inks due to sebum on the fingers and other foreign matter during handling of the lithographic printing plate such as when it is set on a printing machine; and to prevent development of scratches and other flaws on the non-image area or image-area (which repels water and accepts printing ink) during handling of the plate.
The inventors of this invention previously found that a plate protective agent which contained an alkylphenyl type nonionic surfactant having an HLB (hydrophile-lipophile balance) and an anionic surfactant in the oil phase of the conventional emulsion type protective agent had the advantage of not reducing the lipophilicity of the image area of a lithographic printing plate. This finding is the subject of copending Japanese Patent Application No. 148519/78. Follow-up research of the present inventors has revealed that the proposed plate protective agent is unable to satisfactorily prevent a decrease in the lipophilicity of the image area when it is applied to "non-washing development", i.e., wherein a developed printing plate is coated with the protective agent without washing with water.